A polyoxyalkylene glycol is a straight-chain polyether glycol represented by the general formula HO--(CH.sub.2).sub.n O!.sub.m --H (in which m represents an integer of not less than 2, and n represents an integer of not less than 1) terminated by a primary hydroxyl group at both ends thereof and is normally produced by the ring opening polymerization of a cyclic ether. A particularly industrially significant cyclic ether is a polyoxytetramethylene glycol (PTMG) obtained by the polymerization reaction of tetrahydrofuran (THF). PTMG is a straight-chain polyether glycol represented by the general formula HO--(CH.sub.2).sub.4 O!.sub.n --H (in which n is the polymerization degree of the compound represented by an integer of from not less than 2 to not more than 4,000) terminated by a primary hydroxyl group at both ends thereof. PTMG is industrially used as a raw material of an urethane resin-based elastic fiber which must be stretchable or elastic.
In recent years, PTMG has been used as a raw material for the manufacture of thermoplastic polyester elastomer as well. For the application as a raw material of elastic fiber or elastomer, a medium molecular weight PTMG having a number-average molecular weight (Mn) of about from 500 to 3,000 is particularly preferred.
As one of the methods for the synthesis of such a PTMG there has been proposed a process which comprises subjecting tetrahydrofuran to ring opening polymerization in the presence of a composite oxide having a Hammett's Ho index of from -10.0 to 3.0 made up of a mixture of trivalent and tetravalent oxides such as SiO.sub.2 --Al.sub.2 O.sub.3 as a polymerization catalyst and a 1:15 to 15:1 mixture of acetic acid and acetic anhydride to obtain a polytetramethylene glycol acetate polymer (PTME) esterified at both ends thereof, and then subjecting the polymer to an alcoholysis reaction to obtain PTMG.
The foregoing solid acid polymerization catalyst enables an efficient production of PTME having a number-average molecular weight of from 500 to 4,000 within a relatively narrow distribution.
However, these methods are disadvantageous in that the continuous production of PTME involves a remarkable decreasing of the activity of the polymerization catalyst. These methods are also disadvantageous in that they result in the presence of a large amount of acetic acid in the free state in the polymerization system, causing the corrosion of the reaction vessel. Accordingly, PTME cannot be continuously produced by these methods on an industrially favorable basis.